1. Field of the Invention
This invention relates to fine acicular .alpha.-ferric oxide and a process for producing the same by a wet process.
The fine acicular .alpha.-ferric oxide according to the invention is useful as a raw material of magnetic powder for magnetic recording because it is composed of discrete void-free particles with a desirable particle size distribution and aspect ratio. In addition, it is also useful as a transparent red pigment and a raw material of ferrite because of its unique crystal growth direction.
2. Description of the Prior Art
Magnetic iron oxide for magnetic recording should meet the following four fundamental requirements. First, it should be free from voids and have a true high density. Voids cause magnetic poles to occur in the vicinity thereof, resulting in partially disturbed spontaneous magnetization, which has an adverse effect on the magnetic properties. Second, it should have a large aspect ratio (l/d), which is a ratio of the major axis (l) to the minor axis (d) of a particle. The larger the aspect ratio, the smaller the demagnetizing field that occurs in magnetic particles. This leads to a greater coercive force. Third, it should have a narrow particle size distribution. A broad particle size distribution has an adverse effect on the switching field distribution (n-SFD), one of the important properties of magnetic tape. Fourth it should be fine in the case where a high recording density is required of magnetic recording medium.
As mentioned above, the most important and fundamental properties of magnetic iron oxide for magnetic recording are particle size, voids, aspect ratio, and particle size distribution. This is true of acicular ferric oxide as a raw material of magnetic powder.
Acicular .alpha.-ferric oxide is usually produced by the topotactic reaction or dehydration with heating of ferric oxide hydroxide (.alpha.-FeOOH), which undergoes crystal modification while retaining its needle-like shape. The dehydration eliminates 0.5 mol of water from 1 mol of ferric oxide hydroxide, giving rise to crystals containing discrete voids or coalesced voids. The volume of voids accounts for about 30 vol %. Therefore, it such ferric oxide hydroxide is used as a raw material of magnetic powder for magnetic recording, the resulting ferric oxide will also contain voids. This is a hindrance to a high-output magnetic recording medium.
There have been proposed several methods for reducing voids. However, it is theoretically impossible that dehydration forms no voids at all. Even if it should be possible to reduce voids by firing at high temperatures, the firing may present other problems associated with fused particles and poor dispersibility.
For the complete solution of the above-mentioned problems, there have been proposed several improved methods for the direct production of acicular .alpha.-ferric oxide which does not need ferric oxide hydroxide (.alpha.-FeOOH). They give void-free, discrete particles of acicular .alpha.-ferric oxide. Such particles yield acicular .alpha.-ferric oxide which is substantially free from voids and has a high density and good magnetic properties.
In what follows, the prior art technologies reported in literature are evaluated in terms of the fineness of particles, which is expressed in relation to the specific surface area. In the case where literature gives no data of specific surface area, estimated values are used which were calculated knowing that the ratio of an actual value to a theoretical value calculated assuming the particle to be cylindrical is abut 1.5. Thus the estimated value is 1.5 times the theoretical value.
Japanese Patent Publication No. 22416/1980 discloses a method which consists of heating at 100.degree.-250.degree. C. an alkaline aqueous slurry composed of ferric hydroxide, citric acid (or a salt thereof), and an alkali compound. This method is claimed to yield particles having a major diameter of 0.5 .mu.m and an aspect ratio of 7. Japanese Patent Publication No. 4694/1980 discloses a method which consists of heating an alkaline aqueous suspension of ferric hydroxide in the presence of at least one organic phosphonic acid compound. This method is claimed to yield particles having a major diameter of 5 .mu.m and a minor diameter of 0.3 .mu.m. The thus obtained ferric oxide particles have an aspect ratio of about 17, but their specific surface area is smaller than 10 m.sup.2 /g because of their large particle diameter.
The present inventors' investigation suggests that the two conventional methods could yield particles having a specific surface area up to 40 m.sup.2 /g. However, such particles are small in aspect ratio and wide in particle size distribution, and hence unsuitable for use as a raw material of magnetic iron oxide for magnetic recording.
Japanese Patent Laid-open No. 216919/1987 discloses a method for producing acicular .alpha.-Fe.sub.2 O.sub.3 by heating at 80.degree.-250.degree. C. an alkaline aqueous suspension of ferric hydroxide in the presence of an organic compound which forms a complex salt with at least ferric ions, said ferric hydroxide being modified with tin(IV) ions. This method is claimed to yield particles having a major diameter of 0.3 .mu.m, an aspect ratio of 10.0, and a specific surface area of 33.0 m.sup.2 /g. Japanese Patent Laid-open No. 270520/1989 discloses a method which is similar to that just mentioned above but has an advantage over it in not carrying out water washing. This method is claimed to yield particles having a specific surface area of 36.9 m.sup.2 /g, although nothing is reported about the major diameter and aspect ratio of the particles.
Japanese Patent Laid-open No. 50326/1988 discloses a method which consists of heating an alkaline aqueous suspension of ferric hydroxide in the presence of a tetravalent metal compound alone or in combination with a phosphorus compound and/or hydroxycarboxylic acid compound. This method is claimed to yield particles having a major axis of 0.4 .mu.m, as aspect ratio of 7, and a specific surface area of 24.5 m.sup.2 /g. Japanese Patent Laid-open No. 162535/1988 discloses a method which consists of incorporating an aqueous suspension of FeOOH (having pH 7 or below) with a phosphorus compound and subjecting it to hydrothermal treatment at a temperature in the range of 100.degree.-130.degree. C. This method is claimed to yield particles having a major axis of 0.8 .mu.m and a specific surface area of about 12 m.sup.2 /g. The photograph in the literature suggests that the particles have an aspect ratio of about 6-8.
Despite several improvements as mentioned above, no acicular .alpha.-ferric oxide has been produced directly by a wet process which is substantially free from voids and so fine as to have a specific surface area in excess of 40 m.sup.2 /g. In addition, no acicular .alpha.-ferric oxide is available which is substantially free from voids and has an aspect ratio greater than 10 and a specific surface area of 20-40 m.sup.2 /g.